How to use java stream map inside java stream filter

Question

I have 2 arrays and want to make a list of role.getRoleName() only with elements that are in both arrays using streams.

final List<String> roleNames = new ArrayList<>();
roleNames = Arrays.stream(roles).filter(role -> role.getRoleId() 
== Arrays.stream(permissions).map(permission -> permission.getRoleId()));

when I write the above code I get

Operator '==' cannot be applied to 'int', 'java.util.stream.Stream'

I understand the error, but I don't know the solution of how to make the permissions stream in only permission.getRoleId integers.

How about using .equals? and that too on roleId. If permission.getRoleId are integers, why do you plan to collect similar data as List<String>? — Naman
– Naman, Commented Sep 9, 2019 at 8:43
roleId is an int and roleName is string. I want to add roleName in the list — Milan Panic
– Milan Panic, Commented Sep 9, 2019 at 8:44

Andrew · Accepted Answer · 2019-09-09 08:58:12Z

4

There is no way to compare such incompatible types as int and Stream.

Judging from what you've shown, Stream#anyMatch might a good candidate.

roleNames = Arrays.stream(roles)
    .map(Role::getRoleId)
    .filter(id -> Arrays.stream(permissions).map(Role::getRoleId).anyMatch(p -> p.equals(id)))
    .collect(Collectors.toList());

This part Arrays.stream(permissions).map(Role::getRoleId) may be pre-calculated and stored into a Set.

final Set<Integer> set = Arrays.stream(permissions)
                               .map(Role::getRoleId)
                               .collect(Collectors.toSet());

roleNames = Arrays.stream(roles)
                  .filter(role -> set.contains(role.getRoleId()))
                  .map(Role::getRoleName)
                  .collect(Collectors.toList());

edited Sep 9, 2019 at 8:58

answered Sep 9, 2019 at 8:47

Andrew

49.8k15 gold badges99 silver badges145 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Naman · Accepted Answer · 2019-09-09 08:52:08Z

3

What you can do is collect unique roleIds for the array of Permissions into a Set as a computed result and perform a contains check as you iterate through the array of Roles. This could be done as :

final Set<Integer> uniqueRoleForPermissions = Arrays.stream(permissions)
        .map(Permission::getRoleId)
        .collect(Collectors.toSet());
final List<String> roleNames = Arrays.stream(roles)
        .filter(role -> uniqueRoleForPermissions.contains(role.getRoleId()))
        .map(Role::getRoleName)
        .collect(Collectors.toList());

answered Sep 9, 2019 at 8:52

Naman

32.7k32 gold badges240 silver badges385 bronze badges

Collectives™ on Stack Overflow

How to use java stream map inside java stream filter

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related